7th Solvay Conference on Chemistry, 1947

The periodic table that every school child has held contains two series of elements: lanthanides (rare earth elements) and actinides. The series that go from Lanthanum (La, 57) to Lutetium (Lu, 71) and from Actinium (Ac, 89) to Lawrencium (Lr, 103), occupy a square in the IIIB group (Lw was the symbol for Lawrencium until 1963 ). When scientists began producing chemical elements that were heavier than Uranium (i.e. transuranium elements), they thought they were in the process of discovering and isolating homologues to the VIB group, homologues to Manganese (Mn, 25), and the following groups, Uranium (U, 92) being placed at the time in the periodic table beneath Tungsten (W, 74). However, by studying the chemical behaviour of these new elements, Glenn Seaborg suggested in 1944 that a group existed which he called actinides, a group beginning with Actinium, just like the series of lanthanides begins with Lanthanum that gives it its name. Glenn Seaborg did not attend the 1947 Conference on Chemistry devoted to isotopes, but his hypothesis is presented and defended by Frédéric Joliot-Curie, who faces the opposition of several participants, including Friedrich Paneth.